Meet the Foam Nest

Reproductive modes in frogs vary greatly, as do the ways in which they deposit their eggs. The túngara frog, Physalaemus pustulosus (=Engystomops), which is the main focus in my lab constructs a “foam” nest–an adaptive character which I’ve become interested in exploring. Foamy substances are produced by some insects, tunicates, fish and, perhaps most famously, frogs.

Foam nests may be constructed by frogs in trees, underground burrows, on top of water, or nearby water sources. This phenomenon has evolved independently several times in both old and new world frogs, living in tropical and subtropical areas.

To form the nest, a gooey gelatinous substance comes out alongside the eggs, and it’s usually the male who whips the substance into a bubbly foam using his back legs. Biologically, these nests are incredibly resilient and able to withstand mechanical forces and environmental hazards.

Depending on the frog species, as well as individual differences within species, embryos and then tadpoles vary in the amount of time they spend in nests before emerging. The nest is thought to protect tadpoles from predators, and also maintains a moist environment to prevent desiccation. The foam nest may additionally provide the eggs with more exposure to oxygen (a hypothesis I’m currently testing experimentally in the túngara frog).

A member of the family Leptodactylidae, the túngara frog is found in Central and South America, The male constructs the nests on or near temporary pools of water. The structure of the foam nest is shown in figure 2 (note the “cortex” of foam surrounding the eggs, evident in 2c).
(túngara frog nest construction – Video Credit: Amphbiaweb.org)

Nests may be singletons or many nests may be connected to one another. Community building is discussed in detail in a paper from Dalgetty and Kennedy in 2010 where three distinct stages of nest building in the túngara frog are explained: 1) creating a bubble raft 2) main building and 3) termination. By exploiting the community nest, pairs spend less time on phase one. While it is unclear if community building is advantageous for all frogs within a given community, it is likely to be so for late joiners, who can skip the first stage of raft building, leaving the early birds to do more of the work–a classic example of the tragedy of the commons.

Stay tuned to this post in the future, where I’ll provide updates of my experimental findings. In the meantime, check out the papers below to learn more.

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